A conventional automotive air conditioner has used single coolant such as R12. Therefore, the constituent of the coolant circulating in a refrigerant circuit is not changed during the operation of the automotive air conditioner.
Since the cooling efficiency of the automotive air conditioner mainly depends on the capacity of a compressor which is also dependent on the engine speed, each part of the automotive air conditioner, such as the compressor, a condenser or an evaporator should have enough capacity to have a sufficient cooling efficiency even under the condition that the engine speed is not high, an idling condition for instance.
Some residential air conditioners a heat pump for example, use a mixed type coolant, the boiling temperatures of each component being different from the other. The mixed type coolant for the residential heat pump has been used for compensating a heating efficiency when the outdoor temperature is low. In other words, the mixed type coolant of the residential heat pump has not been used for increasing the cooling efficiency or for controlling the cooling efficiency of the air conditioner.
Since the volume of the coolant circulating in the refrigerant circuit of the residential heat pump is much smaller than that of the automotive air conditioner, and since the discharge volume of the compressor used in the residential heat pump is constant, the mixed type coolant used for the residential heat pump could not have been used for the automotive air conditioner.